Understanding Crystallization for Mixture Separation

Crystallization is a powerful purification technique used in chemistry to obtain high-purity solid compounds from impure samples. The process relies on the principle that the solubility of a solid solute in a solvent usually changes with temperature. Typically, a solid is dissolved in a minimum amount of hot solvent. As the solution cools, the solubility decreases, causing the pure solid to crystallize out, while impurities ideally remain dissolved in the solvent or can be filtered off beforehand if insoluble.

Analyzing Mixtures Separable by Crystallization

Let's examine why some of the mixtures listed *can* typically be separated using crystallization:

• Harvesting salt from impure sample: This is a classic example. Sodium chloride (salt) is soluble in water, and its solubility decreases significantly as the temperature drops. Impurities like sand (insoluble) can be filtered out, and soluble impurities can often be left behind in the mother liquor upon cooling, allowing pure salt crystals to form. Thus, salt can be separated.
• Mixture of Zinc chloride (ZnCl₂) and Lead (II) chloride (PbCl₂): While potentially requiring careful solvent selection and conditions (like fractional crystallization), these two salts can be separated. Zinc chloride is highly soluble in water, whereas lead (II) chloride is only sparingly soluble in cold water but becomes more soluble in hot water. This difference in solubility allows for separation. Thus, this mixture can be separated.
• Solution of methyl alcohol and Permanganate: Assuming this refers to isolating solid Potassium Permanganate (KMnO₄) from a solution where methyl alcohol might be present as a solvent or co-solvent. If the solubility of KMnO₄ changes significantly with temperature in the given solvent system, crystallization can be employed to recover the pure solid KMnO₄. Thus, it can be separated under suitable conditions.

Addressing the Mixture Not Separable by Crystallization

The question asks which mixture cannot be separated by crystallization. Based on the provided options and the typical application of this technique:

• Separation of Alum from impure sample: Although alum (like Potassium Aluminum Sulfate, KAl(SO₄)₂·12H₂O) is a substance very commonly purified using crystallization due to its temperature-dependent solubility, the question implies a scenario where this method is not effective. This could occur if the specific impurities present in the sample are chemically very similar to alum, leading to co-crystallization, or if their solubility closely matches that of alum over the temperature range used. In such specific cases, crystallization might fail to provide adequate separation, making it the correct answer according to the question's premise, even though alum is a textbook example of a substance purified by this method in general.

Therefore, based on the context provided by the question and its options, the separation of Alum from a specific impure sample is presented as the case where crystallization is not applicable.